Present communication systems, for example cellular telephone communication systems, operate on narrow or medium bandwidth technologies. The GSM cellular telephone system, a Time Division Multiple Access (TDMA) system, is one such medium bandwidth system. As requirements for higher speed transmission for, for example, data or video transmission increase, wider band technologies are becoming more desirable. One such wideband technology for cellular communications is Wide Band Code Division Multiple Access (WBCDMA). As new technologies develop it is common to provide communications equipment capable of operating on more than one communications system to facilitate the transition from one system to the other or to allow users access to the combined capacity and features of both systems. Thus there is a need for communications receivers capable of receiving signals from both narrow or medium bandwidth systems and also from wide bandwidth communications systems.
Present efforts to provide a single wideband and medium band receiver have resulted in receiver designs which merely duplicate the receiver circuitry for each mode by providing different receiver data paths for each mode. As an example, in the Wideband CDMA system currently being developed in Europe, Universal Mobile Telephone System (UMTS), the channel bandwidth is 3.84 MHz while the current European cellular system, GSM, has a bandwidth of 200 KHz. Both of these systems operate in an IQ modulation mode wherein information to be transmitted, after being appropriately encoded, is provided in In-phase and Quadrature-phase modulation components superimposed on a carrier signal producing a complex signal which is subsequently demodulated and decoded by the receiver to reveal the originally encoded information.
Currently these two modes are combined into one communications receiver unit by providing two different receiver circuits with different downconversion mixers, blocking filters, amplifiers, antialiasing filters, and analog to digital converters. Each of the receiver circuits is thus a separate receiver capable of receiving and decoding the I and Q components of the transmitted signal. Accordingly, it would be advantageous to provide a single communications receiver capable of operating in several modes and in which many elements of circuitry are usable in each mode, thus reducing the complexity of the receiver by reducing the number of circuits necessary in the receiver.
Since the bandwidth of the WBCDMA UMTS system is 3.84 MHz, each I and Q component of the WBCDMA signal will be in the range of 3.84 Mhz/2. These wide bandwidths, given the required dynamic range of the transmitted signal, require the use of sophisticated data converters such as Flash or Pipelined data converters which, while quite fast thus allowing high sampling rates to accommodate code tracking to recover the transmitted code in a spread spectrum system, are not particularly suitable for the GSM mode since these data converters have high power consumption and have limited dynamic range (10 bits) while the GSM mode requires higher dynamic range (14 bits) but allows lower sampling rates. Lower power consumption, of course, is always desirable in portable or mobile communications equipment such as cellular telephones. The GSM mode allows the use of Sigma-Delta digital to analog converters or other converters which are easily programmable for different bandwidths, have wider dynamic range, and consume less power.